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Pierce's Disease has been a scourge of vineyards for well over a century. It was discovered near the end of the 19th century by Newton Pierce, so the disease took on his name after originally being given the name Anaheim Disease (having been discovered in that area’s vineyards well before Disney and all that pavement moved in). Pierce’s Disease (PD) is a nasty disease caused by the bacterium Xylella fastidiosa, which is vectored by several different sharpshooter insects, including blue-green, red-headed and glassy-winged. The bacteria colonize the xylem vascular tissue. The bacterial colonies, along with gums that are exuded by the vine itself, clog the vessels and effectively choke off the vine’s water transport system. The disease is catastrophic in that it cannot usually be surgically removed from the vine and will most often quickly reduce both its fruit production and fruit quality before finishing off the job and killing the vine after a few years.
It is no wonder why so much money has been poured into finding solutions to this disease. Since 2001, the California Department of Food and Agriculture (CDFA) and the Pierce’s Disease and Glassy Winged Sharpshooter (PD/GWSS) Board have directed funding from grower
assessments toward research to combat this devastating disease. Over the years, the assessment has averaged $1.39 per $1,000 of crop value. Solutions have been numerous and diverse. I wrote about them most recently in 2017. Andy Walker of UC Davis has developed several PD-resistant winegrape varieties. Tastings have shown that they can produce good wines and have enough Vitis vinifera in their parentage that they do not show off-characters from the other species in their parentage. I believe Walker’s varieties will have a niche in our industry, serving as blender varieties for non-varietal-labeled wines and as vineyard buffer rows to protect the bulk of the block against sharpshooter movements emanating from Xylella hotspots.
There have been numerous other approaches intended to integrate PD-resistance into existing varieties. All of them involve some sort of transgenic manipulations of the vine, which will likely make it difficult to commercialize those vines under those approaches. However, some of these approaches have been shown to be effective when integrated into rootstocks, as the gene products produced by the introduced genes may move from the rootstock and into the scion. Therefore, it may be possible to transform the rootstocks to protect the scion without the need for a transformed scion. This could be a way around dealing with so-called GMO grape varieties.
What About Biocontrols?
Outside of creating new grape varieties or transforming traditional varieties, can we control PD by introducing a biological agent into the vine to combat the bacteria? In my 2017 article, I mentioned phages, which are bacteria-destroying viruses that can be introduced into the vine to systemically clear the vine of disease. The downside to the phages is that they do not overwinter and would need to be re-applied each year to maintain the protective effect. Non-pathogenic forms of Xylella have also shown promise, and I’ve seen this firsthand in a trial in Riverside, Calif. But that approach has also been inconsistent in trials. Antimicrobial fungi have also been investigated.
The real beauty of some of these biocontrol approaches is that the biocontrol agents can potentially be introduced into the vine topically by using the appropriate penetrating surfactant. A spray-on Xylella killer? That would be far better than pressurized trunk injections, which some treatments would require. Ideally, the PD treatment could be introduced with the first mildew spray of the year, thus eliminating any labor expense.
Somewhat recently, I was introduced to a biotechnology company called Innate Immunity which, in conjunction with a non-profit group called the New Mexico Consortium, has developed a grape-derived peptide that has been demonstrated to clear out a Xylella infestation in a mature grapevine. I’m not the one breaking this news, mind you. It was reported last September that the group had received funding from the CDFA and GWSS board for funding.
I recently spoke with some of the players on the team: Michelle Miller, co-founder and manager of Innate Immunity; Goutam Gupta, senior research scientist of the New Mexico Consortium; and Pete Downs, technical advisor to them for the PD project. They were very optimistic about the effectiveness of their product. Of course, they would be—it’s their brainchild. But their initial results were encouraging enough to garner funding from the board, the first private organization to be recommended for funding by the PD/GWSS board.
Similar peptides have been used for other bacterial pathogens in other hosts. For instance, huanlongbing, or citrus greening disease, is caused by a phloem-mobile bacterium, and peptides have been developed against it. Likewise, peptides against fireblight disease in pome fruits have been developed. These peptides augment the plant’s natural pathogen immune systems, of which plants have limited natural ability to ward off serious disease agents like Xylella fastidiosa.
Preliminarily, the team tested the grape-targeted product in commercial vineyards on vines that were known to be infected by Xylella. About three dozen vines were treated while they were either dormant or just starting to break out of dormancy. The peptide was applied in a spray, along with a commercial adjuvant called Pentra-Bark, which allows the product to move through the tough bark of the vine and into the internal vascular system. So, the product can be applied before the growing season even starts, but it can be applied at other times as well.
They applied the material either once or twice to the vines and provided a control treatment that contained only the Pentra-Bark surfactant without peptide. They later tested the tissues for the presence or absence of Xylella bacteria by using QPCR techniques and found that the peptide-treated vines were cleared of the bacteria. The clearance of bacteria was 100 percent—not partial. That is a rather satisfying result.
I mean, can this peptide cure PD? Early results suggest yes.
The peptides are not completely natural. They are initially chemically synthesized. They then transform tobacco plants to produce the peptides that they have synthesized. So, while the process is not something that can be replicated in nature, the process of using a plant to produce the active ingredient puts this material in the realm of a biopesticide. It’s not considered to be organic, however, so keep that in mind.
With the new CDFA funding, the group will expand testing into a variety of commercial vineyards known to have problems with PD. The first trial will be in Sonoma County, the second in Kern County and the third in Santa Barbara County. The trials will be two years in length, and they have chosen locations with a variety of climates and insect vectors. The goal of these trials is to confirm the efficacy of the products but also to determine the lowest concentration of material that can be effective, the number of applications needed and discover what the longevity is for protection. My guess is that the trials will need to go longer than two years; but after two years, they should know if this approach will have traction or not. I’m hoping it will.
Preliminary toxicity analyses look promising; but if this is to be commercialized, the toxicity testing will need to be much more expansive. The product is in powder form, and considering it is not a living organism, it has a long shelf-life. A patent has been applied for by the company for this product.
Perhaps even more exciting, but not yet in testing phase, is the possibility that this technology could be used against plant viruses, potentially including leafroll and Red Blotch. The peptides target the viral coat proteins and can be delivered into the host plant by using techniques such as nano particles or even virus vectors. I would hate to get overly excited about the idea that this type of approach could be used to clear out viruses from our vineyards, but it sure is an intriguing thought. I mean, could you imagine the applicability of this product if it could do so? I would estimate that about half of our acreage of winegrapes have economically-damaging levels of one or more strains of these two virus diseases, so a spray-on cure would be truly remarkable and a potential savior for the industry. While it is too early to get excited, it is quite an exciting possibility if it ever comes to fruition.
We shouldn’t lose sight of the other approaches being researched and developed to control Pierce’s Disease, like those I mentioned previously. We may very well need a combination of approaches. But this is an exciting development and it sure would be great to have PD in our rear-view mirror someday.
Check out Mark's full article in the August issue of Wine Business Monthly.
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